A light bulb moment on 40/40 freehand grinding

[John Whitley]

A personal light bulb moment I'll share from Stu's new video on sharpening. Around the 14:40 mark he demonstrates a slightly different breakdown to 40/40 gouge grinding than I've seen before. In particular, I've found that I've sometimes over-rotated a 40/40 gouge during sharpening, screwing up the wing angle. I "know" better, but it still happens from time to time.

Stu's teaching approach uses one-directional motions starting from the wing to the gouge tip. For me this is the "lightning bolt": I realized that my struggle with over-rotation is directly related to two-directional sharpening, with motion back towards the wing from the center. It's really easy to screw this up and go just a little too far. And next time, maybe I go a little farther trying to get the (already slightly incorrect) wing fully sharp, and so on. With enough sharpening experience, that's not going to be a problem, but Stu's method should help avoid this entirely. It's a lot easier to maintain correct flute presentation once at the setup for each side. Even better the gouge isn't yet in contact with the wheel at this point so there's no "rush" in getting it right.

(Related, see the recent thread Stuart Batty has released a number of new videos)

 

[john lucas]

I call my 40/40 grind a 40 45. I prefer longer wings than Stewart so I swing the handle further than he does. Mine actually may be closer to a 40/50 grind

 

[Brian Gustin]

Or do a 35-40-50 grind? (I'm thinking of Raffan's asymmetric grind.. I actually tried making my own grind of that type once and it did work quite well used similar to how Raffan does... I just couldnt get comfortable with it and went back to my Ellsworth grind)

 

[John Whitley]

Grind geometry variants are all good, and I use a number myself. But getting an incorrect geometry (and one that ends up being really problematic) unintentionally is much less good.

Regarding setting wing angle in freehand grinding, I accomplish that by the left/right swing of the gouge rather than additional rotation along the axis. Again, I find that that easy to setup and grind consistently. So far, excessive (bowl) gouge rotation (along its axis) mostly just seems to produce a really annoying (and inconsistent) wing for me.

 

[robo hippy]

Well, the difference between a jig ground 40 and a hand ground 40/40 is the wings on the jig method are far more acute. I really don't know how much of a difference that really makes. I roll my flutes from straight up, to 90 degrees on the sides. I do use the nose for most of my cutting. I quit using the swept back gouges after learning to hand/platform grind the 40/40. The mechanics you use for this method are identical to what you do when you turn. Anchor on the tool rest, rub the bevel, and cut. All rotation is with your body, not your arms. I remember seeing Ashley once, and she could only sharpen on one side of the grinder. Seemed strange to me.

robo hippy

 

[John Whitley]

I really don't know how much of a difference that really makes.

Stu's claim, which seems plausible to me from the mathematics and geometric modelling sides, is essentially that a "true" 40/40 elliptical/Vee grind is the only one with the property where the cuts' "aggressiveness" (my term; I forget exactly how Stu puts that) is changed with one motion: rotation about the tool axis. You can see this all the time in his demonstration videos, e.g. closing the flute angle at the end of a cut. It's a pure axial rotation of the tool. For other gouge geometries, you have to perform multiple motions (e.g. both axis and angle-to-the-work changes) for the equivalent operation.

Within some limits however, I'm not clear whether that's a practical consideration. For example, I'd honestly have to go back and forth between turning with "almost" 40/40 and 40/40 grinds to really put out a meaningful comparison. Further, I suspect that point is moot, again within limits because turners learn to make multi-axis movements all the time. I think the main point of this geometry is that it's really good at two things at the same time: the nose slices the end-grain while the wing efficiently peels the side-grain waste. So anything that's "close enough" to have both of those properties, then you're getting the nominal benefits that Stu is after. IIRC, he even mentions in some demonstration videos various "error bars" on the grind angle, wing angle, etc. that are close enough to still function.

 

[robo hippy]

Most of the time when I am using a 40/40 grind, I am cutting more with the nose and less with the wing. I do use them only for finish cuts, and do all of my roughing with the Big Ugly tool. That was why I commented on if having a more acute angle on the wings made any difference or not. The nose cuts at more of a shear angle, and the wing cuts with more of a scraping cut/angle.

I am kind of surprised that no one has invented this yet. To get a simple but 40/40 grind on a gouge, you need some thing like the Wolverine set up, but when you lock the gouge in the jig, it is in a tube or half tube that will pivot so you can get the correct angle on the wings.

robo hippy

 

[Michael Brown]

I was under the impression that the only reason you need to roll the tool while sharpening Stu's 40/40 was because of the wheel (radius). Using a belt sander or the sorby sharpener thing... You can just rotate the gouge on axis and forgo the sweeping.

Not true?

 

[John Whitley]

I was under the impression that the only reason you need to roll the tool while sharpening Stu's 40/40 was because of the wheel (radius). Using a belt sander or the sorby sharpener thing... You can just rotate the gouge on axis and forgo the sweeping.

Not true?

No, not true. The wheel radius is not related to the sharpening motion at all. A flat sharpening system simply means there isn't a hollow grind (which is determined by the wheel radius) to the bevel... but the bevel is usually kept so short for a 40/40 (e.g. 1/8") due to having the heel ground away that this is practically no difference at all.

The "geometric intuition" is that the gouge's motion must keep the flute perpendicular (tangent, really) at the point of contact with the wheel (or belt sander's platen) at all times. This is why, for the required elliptical or vee flute shape, the handle swing is very slow or nonexistent towards the wings but speeds up radically near the tip. Likewise, we're always keeping the bottom of the flute (at the point of contact) parallel to the platform as the handle swings. That determines the rotation of the gouge around its axis. For an elliptical flute, the effective radius at the wings is large (slow rate of change) but the radius is small at the tip (higher rate of change).

 

[John Kananis]

I was under the impression that the only reason you need to roll the tool while sharpening Stu's 40/40 was because of the wheel (radius). Using a belt sander or the sorby sharpener thing... You can just rotate the gouge on axis and forgo the sweeping.

Not true?

No, you have to roll the flute because its parabolic. On a v flute, you sharpen straight across.

 

[robo hippy]

Sweep is necessary to shape the wing. Makes no difference with belt or wheel. On a parabolic flute, a gradual roll. With a V flute like D Way or Thompson, you sweep the nose, then a quick flip over to the wing. If you are getting that bird's beak near the nose, you are spending too much time at that one spot.

robo hippy

 

[Michael Brown]

Sweep is necessary to shape the wing. Makes no difference with belt or wheel. On a parabolic flute, a gradual roll. With a V flute like D Way or Thompson, you sweep the nose, then a quick flip over to the wing. If you are getting that bird's beak near the nose, you are spending too much time at that one spot.

robo hippy

Yeah. I use a wheel and sweep. I just thought I saw someone's YouTube years ago virtuing belt grinders for that reason. Now that I read yours again it makes perfect sense the sweep is always necessary.